1. Field of the Invention
The present invention is related to a method of performing cell reselection procedure in wireless communication system, and more particularly, to a method of performing cell reselection procedure in wireless communication system when a serving cell becomes barred.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) has developed various wireless communication networks. A universal mobile telecommunications system (UMTS) or a global system for mobile communications edge radio access network (GERAN) is a 3rd generation (3G) network which adopts a wideband code division multiple access (WCDMA) technology capable of providing high frequency spectrum utilization, universal coverage, and high-speed/quality multimedia data transmission. In the UMTS network, a radio access network known as a universal terrestrial radio access network (UTRAN) includes multiple cells, commonly known as Node-Bs (NBs), for communicating with multiple user equipment (UE). A long-term evolution (LTE) system is a 4th generation (4G) network which includes a new radio interface and radio network architecture capable of providing a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE network, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple UEs and a core network which includes a mobility management entity (MME), a serving gateway and other devices for NAS (Non Access Stratum) control.
According to 3GPP specifications, after selecting a public land mobile network identity (PLMN), the UE may perform a cell selection procedure and a cell reselection procedure in order to use related network services. The cell selection procedure allows the UE to quickly camp on a serving cell, thereby receiving system information from the PLMN, establishing radio resource control (RRC) links, accessing network via control channels, and receiving/replying paging messages. The cell reselection procedure allows the UE to camp on another target cell having better signal quality than the current serving cell.
After finishing the cell selection procedure, the UE may continuously receive system information from the serving cell, thereby acquiring cell reselection criteria, a list of frequency/radio access technology (RAT) and status information, based on which it may be determined if another target cell having better signal quality exists. Whether the cell reselection procedure should be performed may be determined according to the evaluations on Intra-frequency reselection and Inter-frequency reselection which adopt different parameters as the cell reselection criteria. The evaluation on Intra-frequency reselection uses cell signal strength as a judging condition. If the signal strength of an Intra-frequency target cell remains stronger than the signal strength of an Intra-frequency serving cell longer than a reselection timer value TreselectionRAT, the UE may handover to camp on the target cell by performing the cell reselection procedure.
Each frequency/RAT in the frequency/RAT list is prioritized, and the evaluation on Inter-frequency reselection uses cell signal strength and reselection priority as judging conditions. If the signal strength of a high-priority Intra-frequency target cell remains larger than a first threshold value longer than the reselection timer value TreselectionRAT, the UE may handover to camp on the target cell by performing the cell reselection procedure. If the signal strength of a low-priority Intra-frequency target cell remains larger than a second threshold value longer than the reselection timer value TreselectionRAT and the signal strength of the serving cell remains smaller than a third threshold value longer than the reselection timer value TreselectionRAT, the UE may handover to camp on the target cell by performing the cell reselection procedure. The same-priority Inter-frequency cell reselection may be performed in the same manner as the Intra-frequency cell reselection.
According to related 3GPP specifications (such as TS 36.304), the cells in a wireless communication system may be categorized as suitable cells, acceptable cells, barred cells or reserved cells according to service limitations. The UE may acquire the types of the cells from the status information of a system information block 3 (SIB3) or a system information block 4 (SIB4). According to related 3GPP specifications (such as TS 36.304), the UE is not allowed to camp on a barred cell or on a reserved cell. Therefore, the cell selection procedure and the cell reselection procedure can only be performed on suitable cells or acceptable cells. However, although a serving cell selected by a specific UE during a previous cell selection procedure was a suitable cell or an acceptable cell, the network may somehow set the serving cell as a barred cell or a reserved cell when the UE is currently camping on the serving cell. After that, all other UEs are forbidden to camp on the barred serving cell of the specific UE. Setting the serving cell as a suitable cell, an acceptable cell, a barred cell or a reserved cell does not influence the signal strength of the serving cell. When the UE detects another low-priority Intra-frequency, the signal strength of the serving cell may always remain larger than the third threshold value even if the signal strength of the low-priority Intra-frequency target cell remains larger than the second threshold value longer than the reselection timer value TreselectionRA. Under such circumstance when the serving cell becomes a barred cell, the prior art UE may not be able to handover to camp on the target cell by performing the cell reselection procedure. Therefore, there is a need for a method of performing cell reselection procedure in wireless communication system when a serving cell becomes barred.